In the completion and production of an oil/gas well, it is often sometimes necessary to drill out a plug or other down hole obstruction which was used in the construction of the well. An example of this is so called fracing plugs used in fracing operations that are commonly conducted in shale formations. The fracing plugs are typically used to isolate lateral or horizontal sections of the well bore so that successive, isolated sections can be fraced to stimulate production. However, the fracing plugs must be drilled out so that the oil from the formation can flow to the well head for recovery.
In these types of workover or intervention activities, it is common to use a tubing string to drill out the plugs. In the past, this was conventionally done with work strings of tubing employing so called eight-round or LTC connections. However, in highly deviated or horizontal wells where most fracing occurs, connections employing eight-round threaded components are not sufficiently rugged enough to withstand the changing tension and compression loads that the work/tubing string undergoes.
Recognizing this problem, many operators elect to use a two-step or dual step threaded connection with a metal to metal radial seal which is an integral connection i.e., there is no coupling between the sections of tubing. While two-step, integral connections for tubing work strings are better than coupled eight-round threaded tubing strings, they are not without disadvantages. For one thing, two-step threaded connections are more expensive to manufacture and more expensive to rethread in the event of damage.
Ideally, a tubing string used in the activities described above e.g., drilling out of plugs and other well intervention techniques, would be capable of withstanding high make-up torque and could be made-up and broken-out multiple times e.g., 20 or more times, without any significant reduction in break-out torque. Such a connection would last longer and while in use would be more rugged and able to withstand the tension compression and bending loads placed on the connection especially, for example, in more acute bending modes e.g. 20 degrees per 100 feet. In particular, such a tubing string to would be resistant to the threaded connections backing-off to the point where the string separates.